BikeMonkey Re-PostA most-confirmedly ex-competitive athlete, I. The formative years, athletically, were the overall formative years and I had the benefit of some formal coaching here and there. One might debate the quality but it was certainly coaching.
I run across the later-life convert to running or cycling, now and again, and there is a common theme. The person who "gets serious" about what-have-you. Marathons, USCF type bike racin', etc. Being smart and dedicated people they go out and train a whole bunch and usually get pretty decent. Then, there is the plateau. "I want to qualify for Boston, my times are consistent but I can't get faster". "I want to do group rides but I'm not fast enough". "I got dropped from the Cat 5s".
Intensity.

Everyone has trouble with this idea, the first obvious thing for distance sports is just to go out as hard as you can for most workouts. This is wrong. Once a certain level of competency/fitness has been reached (you completed a marathon? okay, you are there) little benefit is obtained by "doing more long runs" or "training more consistently" or the like. You need to run faster to improve. Speedwork, intervals, etc are the only way.
The cycling plateau is usually the group-ride threshold because being able to stay with the group of riders is a pretty necessary calling card. I can't tell you how many people focus on average speed. "Well I can hold 18mph for my rides but I hear the local group ride is 24mph so I have to ride more so that I can hang". Wrongo. Once you get up to the approx 18mph average on mixed terrain you are ready for the next step. Group rides and yes, you will get dropped at times ( So know your roads). The first reason is , of course, the benefit of drafting. Otherwise known as not having to bash through the wind all by yourself. People know this intellectually, of course, but nothing like a 50 miler in a group ride to really generate understanding. The other reason is subtler. You just can't ride that hard by yourself. Call it motivation, nod to intermittent effort, whatever. There is some weird physiology at work. You'd think all effort would be the same, right? Put out X watts because of a hill, increased wind resistance, or drag brake and it should all be the same training, no? Dunno why but it doesn't seem to work this way. There is no substitute for sustained big gear riding that you can only maintain because of the pack. So you have to suck it up and go on those local group rides. You'll get dropped at first, perhaps frequently. Eventually, you'll develop the skills and power and notice you are a much better rider. You won't get there by yourself no matter how many hours you put in.
This has something to do with science careers.

Normally I would point and laugh at your hilarious appeal to authority, but you are pretty on the money here. This is exactly what is missing from my current training plan and is perhaps the reason I am bored.

OK, here's what it has to do with science: most of us need to get away and flush our brains daily. Except, that doesn't really happen - you go home, twiddle around, have dinner, watch TV, and intermittently you're still thinking about, that's right, science.
Ditto for a solo ride: you don't ride that hard by yourself, see above. Your mind still has time to wander back to the lab.
On a group ride when you're getting your legs ripped off and your lungs are back at the last stop sign, your brain has no time at all to be thinking about science. As hard as that is, it's good for you the next day. Flush!